Timing-based servo (TBS) is a technology developed for linear tape drives. In TBS systems, recorded servo patterns consist of transitions with two different azimuthal slopes. Head position is derived from the relative timing of pulses, or dibits, generated by a narrow head reading the servo patterns. TBS patterns also allow the encoding of additional longitudinal position (LPOS) information without affecting the generation of the transversal position error signal (PES). This is obtained by shifting transitions from their nominal pattern position using pulse-position modulation (PPM). A specification for the servo format in current midrange tape drives is provided by the linear tape-open (LTO) format. The complete format for LTO drives of generation 1 (LTO-1) was standardized by the European Computer Manufacturers Association (ECMA) in 2001 as ECMA-319. Additional information on LTO technology, in particular on LTO drives of generations 2 to 4 (LTO-2 to LTO-4), where the servo format was not modified, can be found on the World Wide Web (www) at ultrium.com. Traditionally, the detection of LPOS information bits is based on the observation of the shifts of the arrival times of the dibit peaks within the servo bursts at the servo reader output (R. C. Barrett, E. H. Klaassen, T. R. Albrecht, G. A. Jaquette, and J. H. Eaton, “Timing-based track-following servo for linear tape systems”, IEEE Transactions on Magnetics, Vol. 34, Issue 4, Part 1, pp. 1872-1877, July 1998). Detection of LPOS bits may also be performed by a matched-filter detector (G. Cherubini, E. Eleftheriou, R. Hutchins, and J. Jelitto, “Synchronous Servo Channel for Tape Drive Systems,” U.S. Pat. No. 7,365,929 issued Apr. 28, 2008).
In tape drives, two dedicated servo channels are normally available from which LPOS information and PES can be derived. One of the two available servo channels is initially selected to provide the LPOS information and PES. The initial selection is maintained as long as satisfactory channel operation is observed. If the performance of the selected servo channel deteriorates, the drive switches to the other servo channel to obtain the required LPOS information and PES. In an alternative method, the two servo channels provide reliability information about LPOS symbols and at the end of each servo frame, the LPOS symbol having the highest assigned reliability is chosen. However, the signal from one servo reader may occasionally fade while the signal from the other reader might not be affected. In such an event, the servo channel affected by fading loses timing and needs to undergo re-acquisition of the channel parameters. Consequently, additional latency and loss of information is experienced.